The present invention relates to the field of optical metrology. More specifically, it provides a method by which reflectance data may be accurately calibrated. In one embodiment it provides a method by which broad-band vacuum ultraviolet (VUV) reflectance data may be accurately calibrated. Additionally, it also provides a method by which highly accurate thin film measurements may be performed.
Optical reflectometry techniques have long been employed in process control applications in the semiconductor manufacturing industry due to their non-contact, non-destructive and generally high-throughput nature. The vast majority of these tools operate in some portion of the spectral region spanning the deep ultraviolet and near-infrared wavelengths (DUV-NIR generally 200-1000 nm). The push towards thinner layers and the introduction of complicated new materials have challenged the sensitivity of such instrumentation. As a result, this has necessitated an effort to develop optical reflectometry equipment utilizing shorter wavelengths (below 200 nm), where greater sensitivity to subtle changes in material properties may be realized. One approach to performing such measurements is described in U.S. application Ser. No. 10/668,642, filed on Sep. 23, 2003, which discloses a system and method for a vacuum ultraviolet (VUV) reflectometer, the disclosure of which is incorporated herein by reference.
To obtain meaningful quantitative results from reflectometry data it is desirable to normalize or calibrate measured reflectance values in order to generate absolute reflectance spectra. At longer wavelengths in the DUV-NIR region this has traditionally been accomplished using a variety of techniques.
Due to the complexity of absolute reflectometer systems, commercial reflectometers generally measure reflected intensity, which is calibrated to a known absolute reflectance standard. In the DUV-NIR wavelength range, a silicon wafer (with native SiO2 layer) is typically used as the optical properties are well-known and the reflectance fairly stable over this wavelength range.
The precise calibration steps vary from instrument to instrument, but in essence the quantity usually measured is
                    R        =                              I            r                                I            0                                              eq        .                                  ⁢        1            where Ir is the intensity reflected from the sample and measured by the detector, and I0 is the incident intensity. I0 is generally not known. In addition, I0 will change over time due to environmental changes, drift in the optical system caused by environmental changes, and to drift of the intensity profile of the light source. At any given point in time, I0 is determined by a calibration procedure:
                              I          0                =                              I            cal                                R            cal                                              eq        .                                  ⁢        2            where Ical is the measured intensity of the calibration standard, and Rcal is the assumed reflectance of the calibration standard. If enough information about the calibration sample is known, e.g. optical properties, surface roughness, etc., then Rcal can be generated using standard thin film models. Subsequent measurements are performed calibrated using this I0 via eq. 1.
This procedure as usually implemented assumes that changes in Ical are due only to the environmental or lamp intensity changes mentioned above, and not due to changes to the calibration standard itself. In fact, variations in the calibration standard over time are generally not detectable using the above method, since such changes are simply “calibrated out”. Obviously, the accuracy and stability of all subsequent reflectance measurements is highly dependent on the accuracy of the assumptions used to generate Rcal, as well as the stability of the calibration sample itself over time.
Some calibration techniques involve complicated optical arrangements that incorporate moving mirrors. Examples of such methods are provided in U.S. Pat. No. 4,368,983 (and references incorporated therein) which describes an apparatus and method to measure the absolute reflectivity of a sample using a multiple pass reflectometer.
While such methods offer a means of obtaining calibrated reflectance data, they generally suffer from the fact that they are time-consuming, involve considerable mechanical motion and can not easily be integrated into systems suitable for use in semiconductor manufacturing environments. Furthermore, many of these methods were designed for use in single wavelength reflectometers wherein a single wavelength detector is used in combination with a wavelength selecting pre-monochromator.
Ideally, it would be desirable to provide a technique by which broad-band reflectometry data could be simultaneously calibrated quickly and simply and in a manner that would lend itself suitable for use in semiconductor manufacturing environments.
One calibration approach is presented in U.S. Pat. No. RE 34,783 wherein a method is described that involves measuring the reflectance from a calibration sample whose absolute reflectance is well known, dividing the measured value by the absolute value to obtain a system efficiency coefficient and then, without changing the illumination or optics, measuring the reflectance of an unknown material and applying the coefficient to the measured value to obtain its absolute value.
In practice, single crystal silicon wafers are commonly employed as calibration samples since they are readily available, controllably manufactured and their optical properties in the DUV-NIR region have been well characterized. This approach works reasonably well at wavelengths above ˜250 nm where the reflectance of single crystal silicon is both stable and predictable.
At shorter wavelengths (<250 nm) the reflectance of single crystal silicon wafers is neither stable nor predictable. Subtle variations in the thickness of the naturally (or “native”) formed silicon dioxide layer present on the wafer can significantly influence the measured reflectance. Additionally, ultra-thin layers of moisture and/or hydrocarbons are known to adsorb onto the surface further modifying the sample reflectance in this spectral region. As a result, it is generally not advisable to regard the reflectance of single crystal silicon wafers at wavelengths <250 nm as a “known” property.
One approach to overcoming this problem is presented in U.S. Pat. No. 5,798,837, which describes an optical measurement system that includes a reference ellipsometer and at least one non-contact optical measurement device, such as a reflectometer. The reference ellipsometer is used to determine an optical property of the calibration sample. The optical measurement device is then calibrated by comparing the measured optical property from the optical measurement device to the determined optical property from the reference ellipsometer.
Integration of a separate reference ellipsometer into an optical measurement system in order to calibrate the first optical measurement device is both complicated and expensive. Furthermore, the reference ellipsometer itself must be properly aligned and calibrated if it is to yield accurate results.
It follows that it would be highly desirable to develop a means of quickly and accurately calibrating broad-band data from an optical reflectometer operating at wavelengths <250 nm without the complication and expense associated with incorporating a second reference instrument into the system.
Additionally, it would be advantageous if this method specifically enabled the accurate calibration of reflectometry data at wavelengths encompassing the VUV spectral region, where small uncertainties in the properties of third party certified standards can result in substantial errors. It would be further desirable if this method was capable of independently determining the properties of such standards so as to reduce or altogether remove the need for their procurement and maintenance.
In addition to providing a technique to enable accurate calibration of reflectometry tools, it is desirable to provide a technique by which highly accurate thin film measurements may be performed. Optical reflectance measurements are used in a wide range of thin film applications. Ordinarily the absolute reflectance of a sample is recorded and subsequently analyzed using mathematical models in order to determine an assortment of physical properties.
Typically, the analysis is deemed complete when a quantitative indicator (generally referred to as the “goodness of fit” parameter) attains a specific value. Unfortunately, there are limits to the measurement accuracy that can be attained using conventional “goodness of fit” parameters. Hence, it follows that it would be desirable to develop a more sensitive measure of “goodness of fit” in order that higher levels of accuracy in thin film measurement may be obtained.